trilobite

Trilobites are extinct marine arthropods in the class Trilobita, dominant in Paleozoic oceans for hundreds of millions of years. First appearing in the early Cambrian about 521 million years ago, they were among the most diverse groups of ancient marine invertebrates. They inhabited a wide range of marine environments and persisted until the end-Permian mass extinction around 252 million years ago.

Trilobites had calcified exoskeletons divided into three longitudinal lobes: a central axial lobe and two pleural lobes. The body comprised the cephalon (head), thorax, and pygidium (tail). The cephalon often bore compound eyes—either holochroal or schizochroal types—faceted by multiple lenses. The thorax consisted of articulated segments enabling flexibility, while the pygidium formed from fused segments. The exoskeleton was shed as they grew.

Most trilobites were benthic, living on or within sediment and feeding by scavenging, detritivory, or predation, with a wide range of forms from small and delicate to large and spiny. Some could enroll for protection. Their fossils record diverse lifestyles and habitats across continents.

Because of their durable shells, trilobites are important index fossils for Paleozoic rocks, especially in the Cambrian and Ordovician. They provide insight into early arthropod evolution and Paleozoic marine ecosystems, and their rich, well-preserved fossil record supports biostratigraphic dating and paleoenvironmental reconstructions.